1. Field of the Invention
This invention relates to the secondary recovery of petroleum from subterranean formations; in particular, this invention concerns an improved water flooding process.
In general, water flooding consists of introducing an aqueous medium into one or more injection wells which penetrate a partially depleted oil-bearing formation: the water flooding medium is forced through the formation toward one or more production wells which likewise penetrate the formation. In the so-called "five-spot drive," the flooding medium is forced down four injection wells which are symetrically located around a production well. The flooding medium passes through the formation displacing the residual oil present therein and carrying it into the production well from which it is recovered by conventional means.
Unfortunately, oil-bearing formations are generally of very non-uniform permeability. The formations usually contain numerous pockets, cracks and other physical characteristics which result in widely varying formation permeability relative to the flow of aqueous media and oil. Thus, when conventional water flooding is undertaken the flooding medium tends to follow a path of least resistance bypassing a good part of the oil present in the formation. As described in the patent art listed below, two methods may be employed for dealing with this problem.
The first of these is concerned with the water flooding operation at the point of the input well. Various viscosity-increasing additives may be introduced into the aqueous water flooding medium in order to establish a more or less planer front with which to sweep the oil from the subterranean formation and into the production well. The formation of so-called "fingers" of flooding medium extending through the zones of highest permeability may thus be reduced by increasing the viscosity of at least the forward portion of the flooding medium to near that of the oil to be recovered or greater. Significant increases in oil output may be achieved in this manner.
The second means for dealing with the non-uniform permeability of the subterranean formations entails the treatment of the production well. The intent of this process is to plug the channels of high permeability (and generally low oil content) thereby forcing the flooding medium which is introduced at the input wells to sweep through the high oil-bearing portions of the formation. An aqueous high viscosity treating medium is introduced at the production well in order to accomplish the blockage of the high permeability areas. Significant improvement in the oil/water ratio obtained at the production well may be achieved in this manner.
Our invention entails the discovery of an improved medium for use in water flooding operations in accordance with the practices just described. The improved treating medium of our invention may be introduced at production wells as well as at input wells.
2. Description of the Prior Art
1. U.S. Pat. 2,827,964; B. B. Sandiford (Union Oil); Issued Mar. 25, 1958.
A water flooding process for recovering petroleum from subterranean formations is disclosed using viscosity-increasing additives in the form of water-soluble, partially hydrolyzed acrylamide polymer. The acrylamide polymers employed include homopolymers of acrylamide as well as water-soluble copolymers of acrylamide with up to 15% by weight with other polymerizable vinyl compounds such as alkyl esters of acrylic and methacrylic acids and methacrylamide. Hydrolysis of the acrylamide polymers entails the conversion to carboxyl groups of between about 0.8-10% of the acrylamide amide groups. The examples in this patent use "typical partially hydrolyzed acrylamide polymers."
2. U.S. Pat. No. 3,039,529; K. R. McKennon (Dow Chemical); Issued June 19, 1962.
Water flooding process for the recovery of petroleum from subterranean formations is disclosed using viscous aqueous compositions containing polyacrylamides of molecular weights of at least 500,000 and preferably 1,000,000 or more wherein from 12 to about 67% of the original carboxamide groups have been hydrolyzed to carboxyl groups. The polyacrylamides employed may be homopolymers of acrylamide or water-soluble copolymers of acrylamide with up to about 10% by weight of other suitable polymerizable vinyl compounds such as vinyl acetate, acrylonitrile, methacrylonitrile and the like.
3. U.S. Pat. No. 3,087,543; H. S. Arendt (Jersey Production Research); Issued Apr. 30, 1963.
Method for preferentially reducing or eliminating water production from an oil or gas producing subsurface formation by treating the producing well with an acrylamide carboxylic acid copolymer is taught. The test examples disclosed in this patent employ Separan 2610 (Dow Chemical product) which is believed to be obtained by the hydrolysis of acrylamide. (See No. 4 below.)
4. U.S. Pat. No. 3,308,885; B. B. Sandiford (Union Oil); Issued Mar. 14, 1967.
Treatment of subterranean hydrocarboniferous formations in order to decrease the water/oil ratio of the total well effluent and to increase the production rate of hydrocarbon fluids is taught. The invention entails the injection, through a producing well, of an aqueous solution of a high molecular weight (at least 200,000 and preferably 1,000,000) water-soluble polyacrylamide wherein at least 8% and not more than 70% of the amide groups have been hydrolyzed to carboxylic acid groups.
The invention is distinguished from that aspect of the water flooding process in which either treated or untreated water is used to "sweep" oil from partially depleted wells. The Sandiford invention rather is likened to Arendt (U.S. Pat. No. 3,087,543; see No. 3 above) in that beneficial effects accrue only while the treating agent is present in the formation surrounding the producing well bore.
The patent claims that the hydrolyzed polyacrylamide disclosed therein is "much more effective in reducing water permeability than the acrylate-carboxylic acid copolymer disclosed in the Arendt patent." The patentee further promises to show this to be the case at a point further on in the specification, although no such demonstration is apparent. Furthermore, the copolymer utilized in Arendt is described in Arendt merely as "an acrylamide carboxylic acid copolymer"; the only copolymer utilized in Arendt is Separan 2610 which, it is believed, is produced by the hydrolysis of acrylamide rather than by copolymerization as suggested in Sandiford.
5. U.S. Pat. No. 3,343,601; D. J. Pye; (Dow Chemical); Issued Sept. 26, 1967.
Water flooding method directed toward inhibiting ferric hydroxide plugging is disclosed which entails the use of an aqueous injection fluid containing a small amount of a water-soluble hydrosulfite and an optional polymeric additive such as a homopolymer of acrylamide or a copolymer thereof with acrylic acid.
6. U.S. Pat. No. 3,721,295; L. L. Bott (Nalco Chemical); Issued Mar. 20, 1973.
Water flooding process for the recovery of petroleum from subterranean formations directed toward reducing the amount of water recovered from the producing well by introducing a water-in-oil emulsion containing 0.01-35% by weight of a finely divided water-soluble vinyl addition polymer is described. The water-soluble vinyl addition polymers include acrylamide polymers such as polyacrylamide and its water-soluble copolymeric derivatives such as acrylamide-acrylic acid salt copolymers. The polymeric latex is generally employed as a concentrate which is diluted with an organic liquid just prior to use.
The water-in-oil emulsion is inverted in the presence of water in the subterranean formation; upon inversion, the polymer goes into aqueous solution forming a uniformly viscous solution. The rate of inversion of the latex may be increased through the incorporation of a surfactant into the latex.
7. U.S. Pat. No. 3,724,547; L. L. Bott (Nalco Chemical); Issued Apr. 3, 1973.
Water flooding process is described which utilizes the composition described in U.S. Pat. No. 3,721,295 (No. 6 above) for recovering oil from subterranean oil-bearing formations. The two patents should be distinguished in that U.S. Pat. No. 3,721,295 is concerned with reducing the amount of water recovered from the producing well whereas U.S. Pat. No. 3,724,547; is concerned with "sweeping" the oil from the subterranean oil-bearing formation.
8. U.S. Pat. No. 3,779,316; L. L. Bott (Nalco Chemical); Issued Dec. 18, 1973.
Water flooding process for the recovery of petroleum from subterranean formations directed toward reducing the amount of water recovered from the producing well by introducing a stable liquid dispersion containing a water-soluble anionic vinyl addition polymer and a water-soluble cationic polymer is described. The water-soluble vinyl addition polymers include the homo- and copolymers of acrylic acid and the water-soluble salts thereof, such as polyacrylic acid-sodium salt; the water-soluble cationic polymers include alkylene polyamines such as ethylene dichlorideammonia condensation polymers, and polyethylene imine.
9. U.S. Pat. No. 3,780,806; L. L. Bott (Nalco Chemical); Issued Dec. 25, 1973.
Water flooding process is described for recovering oil from subterranean oil-bearing formations through the use of the stable liquid dispersion of U.S. Pat. No. 3,779,316 (No. 8 above). This patent differs from the U.S. Pat. No. 3,779,316 patent in that the liquid dispersion is being used to sweep oil from the oil-bearing formation rather than to reduce the amount of water recovered from the producing well.
10. U.S. Pat. No. 3,893,510; E. A. Elphingstone (Halliburton); Issued July 8, 1975.
In a water flooding process for recovering petroleum from subterranean formations, the use of high molecular weight polymers to reduce the permeability of the oil-bearing formation to the flow of water is described. Suitable water-soluble polymers for dispersion in the water-in-oil emulsion include polyacrylamide and copolymers of acrylamide and sodium acrylate.